Refactored build scripts to support TB2, TB3 + integration tests (#12572)

Refactored build scripts to support TB2, TB3 + integration tests (#12572)

* Main build configuration triggers unit tests for all modules and uitest/build.xml testbench tests in parallel
* uitest/build.xml triggers Jetty startup and integration (server) tests in parallel. After the server has started, TB2 and TB3 tests are run in parallel.
* Server integration tests for servlet containers are run using TB3 and the com.vaadin.tests.tb3.ServletIntegrationTests test suite.
* Portlet integration tests are still run using TB2 test scripts

Change-Id: Ie6bffd4e68b4889074e9c470faa3c65f923e55c4